Thread: Converting Inches of water to FPM and CFM

your measured 2" of WC is the pressure between what?

The only time a manometer can be used easily and reliably to get an estimate of the CFM is under what is called a static condition i.e. when the DC is running but there is zero flow i.e. all the blast gates are closed. One end of the manometer can be placed anywhere inside the ducting and the other outside the ducting.

Then use the equations or chart in this post
https://www.woodworkforums.com/f200/d...2/#post1679728

This will provide an indication of the max flow rate of your ducting system for a specific duct diameter.
However it won't tell you what will be flow will be if the filters are blocking up, or the ducting is resistive or if a machine especially is restricting the air flow.
Even a naked duct opening adds resistance and won't have that max flow unless something like a Bell mouth hood is used on the end

You can measure the pressures under flow or under "dynamic conditions" (i.e. air is moving in a duct or through a machine) for comparative purposes only.
To get an accurate assessment of the flow under dynamic conditions
- a calibrated pitot tube and a manometer, or an air flow meter, are required. Pitot tubes have two specific connection points on the tube itself to which the manometer is connected.
- the devices in the previous point must be located inside a length of test duct that is at least 5 clear (i.e. no junctions or gates) duct diameters long either side of the measuring point.
- the pressures are measured at a number of points across the diameter of the test duct and integrated mathematically.

Using a manometer under dynamic conditions without the above may provide an indication of relative improvement
The greater the pressure differences the smaller the flow.
So if you want to see if your machine is breathing properly and the pressure inside the duct following the machine is X", and then you change something about the air path and at the same point it becomes Y".
If Y" is less than X" then you have more air flow. If Y">X then you have made things worse.

Thanks for the clarification Bob. I was of the incorrect opinion that one could get pretty reasonable readings at the machine with a manometer.

I didn't get 2"WC out of anything, it was just an example.

Originally Posted by ozhunter

Thanks for the clarification Bob. I was of the incorrect opinion that one could get pretty reasonable readings at the machine with a manometer.

I didn't get 2"WC out of anything, it was just an example.

Please bear in mind that calcs done from static pressure are a guide only.

Two impellers might give a similar static pressure, but one may be more effective than the other when actually pulling air and overcoming restrictions such as the machine, its hoods and the ducting/fittings.

Have fun!

John

Originally Posted by John Samuel

Please bear in mind that calcs done from static pressure are a guide only.
Two impellers might give a similar static pressure, but one may be more effective than the other when actually pulling air and overcoming restrictions such as the machine, its hoods and the ducting/fittings.

Along the same lines, just because an impeller (say from a vacuum cleaner) is able to generate 30" of WC that doesn't mean it will draw more air than a DC that generates only 10" of WC. The impeller has to be of a specific size to physically be able to transfer the air. The small impellers used in VCs are typically limited to about 100 CFM almost irrespective of the duct size, whereas a 12" impeller on a DC can in theory move about 10 times more air but it requires a 6" duct with low back resistance to enable it to do so.

I'm curious how much does altitude reduce suction? A dust collection system at sea level has to be better than at 2000 meters / 6,000 feet for example.

Pete

Originally Posted by QC Inspector

I'm curious how much does altitude reduce suction? A dust collection system at sea level has to be better than at 2000 meters / 6,000 feet for example.

Tis indeed an interesting question

Short version:

a) For an unrestricted motor/impeller the swept volume depends on the RPM and impeller size/design so there is no change in the maximum volume of air moved.

b) For a real DC system the volume of air moved depends on a), and the back pressure/resistance of the ducting/machines

The back pressure of the ducting depends on the a bunch of things (diameter and roughness of ducting, viscosity of air and the density of the air)

The density of the air at 2000 m is 80% of that at sea level.

Assuming everything is the same about a DC system, plugging everything into a pressure drop calculator and it turns out the DC will move about 10% MORE air volume at 2000 m than at sea level.

Of course the air density is 20% less so the total number of air molecules moved will still be about 10% less than at sea level.
Since the air is less dense, the air flow
1) cannot slow down big chips and force them into the collected air stream as easily,
and
2) suspend the same amount of sawdust is the air can it can at sea level.
How significant this effect would be is hard to say. 2) is only really relevant for high dust loads

My conclusions
Because more air volume will be collected at source, fine dust extraction should be enhanced by about 10%, but OTOH the fine dust will diffuse more easily from the source it will probably work out to be about the same.
Large chip collection performance will probably be reduced.

Thanks for the fast reply to my question Bob. I thought higher altitudes would have equal effects on a dust system but it looks like we need to do our surfacing and thickening at sea level and the sanding in the mountains, perhaps with the sawing somewhere along the way?

Pete